VITAMINS MAY CUT BIRTH DEFECT RISK

By GINA KOLATA

Published: December 2, 1988

Women who take multivitamin pills at the time of conception have less than half the risk of having a baby with a serious neurological defect as do women who are not taking these vitamins, a new study has found.

The work supports previous findings indicating that vitamins taken at the time of conception can prevent these neural tube defects. They can range from spina bifida or open spine, which can cause paralysis, to anencephaly, a lethal condition in which most of the brain is missing. About one baby in 1,000 is born with a neural tube defect.

The study is published in today's issue of The Journal of the American Medical Association.

Dr. Joseph Mulinare of the Centers for Disease Control, an author of the new study, said he is waiting for more data from other studies before advising women on taking vitamins at the time of conception.

''It would be premature to make that recommendation,'' he said, because the study is not conclusive. But he added, ''People do think multivitamins are pretty harmless.'' Danger of Extra Doses

Other experts said they werre already advising women to take multivitamins before trying to become pregnant and that they hoped the new study would encourage more doctors to give this advice. But they stressed that it was unsafe to take extra doses of vitamins because some vitamins can cause birth defects when taken in large quantities.

The Spina Bifida Association of America, which supports research on neural tube defects and counsels families about these birth defects, makes a point of advising women that there is increasing evidence pointing to an association between vitamin use and a reduced risk of neural tube defects.

''There is absolutely no evidence of any harm from taking multivitamins,'' said Dr. Mark I. Evans of Wayne State University in Detroit. ''I strongly advise all women in the childbearing age group to do so.'' He said he hoped the new study ''would encourage more obstetricians to give this advice.''

Pregnant women are routinely tested to see if their fetuses have neural tube defects. Women carrying fetuses with spina bifida are advised to have Caesarean deliveries to minimize trauma to the baby's spinal cord during birth. Many women choose to abort affected fetuses. Since the use of vitamins would not eliminate the problem of defects, doctors would continue to offer the prenatal tests.

The new study involved a comparison of 347 women in the Atlanta area who gave birth to babies with neural tube defects and 2,829 women whose babies did not have birth defects. Dr. Mulinare and his colleagues found that the risk of having a baby with these defects was 60 percent less for women who took vitamins before and at the time of conception than it was for those who did not take vitamins.

The researchers said there may have been some other factors involved. But the findings are consistent with several other lines of evidence indicating that one cause of neural tube defects might be vitamin deficiencies.

For years, researchers have speculated that a pregnant woman's diet might have something to do with neural tube defects. These birth defects arise early in pregnancy, at the end of the first month after conception. At that time, the neural tube, which later forms the spinal column, starts to close, like a zipper, from the middle of the fetus' back. If it fails to close properly at the top, the fetus has anencephaly. If the tube fails to close toward the bottom, the fetus has spina bifida, a condition that can result in partial paralysis. Detection of Defects

Neural tube defects can be detected prenatally because a fetal protein pours out of the opening of the spinal cord and enters the mother's blood.

To test for the birth defects, technicians first look for the fetal protein, alpha fetoprotein, in the pregnant woman's blood. Women with high levels of the protein in their blood are referred for sonograms and those with suspicious sonograms are referred for amniocentesis, in which doctors examine the amniotic fluid that surrounds the fetus and confirm the high levels of alpha fetoprotein.

Neural tube defects are more likely to occur in babies born to women of lower socioeconomic classes and women who have poor diets for other reasons, researchers say. ''There was an epidemic of neural tube defects during the Depression,'' Dr. Evans said, ''and the incidence has been falling ever since.''

In 1976, Dr. Richard W. Smithells of the University of Leeds in England found that women who had a babies with neural tube defects had lower levels of folic acid and vitamin C in their blood than women whose babies were normal. Reasoning that women who have had one baby with a neural tube defect have a 5 percent chance of having another, a fifty-fold increase in risk, Dr. Smithells decided to try to prevent these birth defects by giving vitamins to these high-risk women.

In a study published in 1981, Dr. Smithells reported that these women reduced their risk seven-fold if they took vitamins. But his study was criticized because it did not include a control group of women who took inert substances rather than vitamins.

Dr. Nicholas Wald of St. Bartholomew's Hospital in London is now coordinating such a study, which will involve about 2,000 women in seven countries. All will have previously given birth to a baby with a neural tube defect.